Foundry Configuration Guide for the FESX, FSX, and FWSX
21 - 4 © Foundry Networks, Inc. December 2005
sessions with them if the router does not have any route information to send in an UPDATE message. See “BGP4
Message Types” on page 21-5 for information about BGP4 messages.
How BGP4 Selects a Path for a Route
When multiple paths for the same route are known to a BGP4 router, the router uses the following algorithm to
weigh the paths and determine the optimal path for the route. The optimal path depends on various parameters,
which can be modified. (See “Optional Configuration Tasks” on page 21-21.)
1. Is the next hop accessible though an Interior Gateway Protocol (IGP) route? If not, ignore the route.
NOTE: The device does not use the default route to resolve BGP4 next hop. Also see “Enabling Next-Hop
Recursion” on page 21-26.
2. Use the path with the largest weight.
3. If the weights are the same, prefer the route with the largest local preference.
4. If the routes have the same local preference, prefer the route that was originated locally (by this BGP4 Layer
3 Switch).
5. If the local preferences are the same, prefer the route with the shortest AS-path. An AS-SET counts as 1. A
confederation path length, if present, is not counted as part of the path length.
6. If the AS-path lengths are the same, prefer the route with the lowest origin type. From low to high, route origin
types are valued as follows:
• IGP is lowest
• EGP is higher than IGP but lower than INCOMPLETE
• INCOMPLETE is highest
7. If the routes have the same origin type, prefer the route with the lowest
MED. For a definition of MED, see
“Configuring the Layer 3 Switch To Always Compare Multi-Exit Discriminators (MEDs)” on page 21-31.
0BGP4 compares the MEDs of two otherwise equivalent paths if and only if the routes were learned from the
same neighboring AS. This behavior is called deterministic MED. 0Deterministic MED is always enabled
and cannot be disabled.
In addition, you can enable the Layer 3 Switch to always compare the MEDs, regardless of the AS information
in the paths. To enable this comparison, enter the always-compare-med command at the BGP4
configuration level of the CLI. This option is disabled by default.
NOTE: By default, value 0 (most favorable) is used in MED comparison when the MED attribute is not
present. The default MED comparison results in the Layer 3 Switch favoring the route paths that are missing
their MEDs. 0You can use the med-missing-as-worst command to make the Layer 3 Switch regard a BGP
route with a missing MED attribute as the least favorable route, when comparing the MEDs of the routes.
NOTE: MED comparison is not performed for internal routes originated within the local AS or confederation.
8. Prefer routes in the following order:
• Routes received through EBGP from a BGP4 neighbor outside of the confederation
• Routes received through EBGP from a BGP4 router within the confederation
• Routes received through IBGP
9. If all the comparisons above are equal, prefer the route with the lowest IGP metric to the BGP4 next hop. This
is the closest internal path inside the AS to reach the destination.
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